1. Field of the Invention
The prsent invention relates to production of free cutting steels, more particularly a method for producing steels containing low melting point metals, such as free cutting steels.
2. Description of the Related Art
Free cutting properties of steels have been conventionally achieved by adding various elements which give the steel required free cutting properties. In particular, addition of low melting point metals, such as lead and bismuth, to free cutting steels containing relatively large amounts of sulfur, phosphorus, calcium etc. can further improve the free cutting properties of these steels, yet with less possibilities of lowering their mechanical properties.
The conventional method for adding these low melting point metals to molten steel has been such that granules of these addition metals are blown into molten steel or into a stream of molten steel by utilizing solid-gas pressure transfer system using inert gas such as argon gas. However, this conventional method has been confronted with a problem that much of the addition metal in gaseous state escapes together with the exhaust of the blowing gas, hence causing vigorous smoking.
The above problem is caused by the fact that these low melting point metal elements have their boiling point temperature around the temperature of the molten steel to which they are added. In the case of Pb, excessively added Pb, due to its large specific gravity and small solubility into the molten steel, precipitates in the molten bath to produce residues containing a large amount of Pb. Also various complicated considerations are required for treating the Pb containing residues.
Further, in order to obtain consistency in the production quality and production yield etc., it is important to achieve an accurate and uniform dispersion of the additive metals into the molten steel bath.
For these purposes, the present inventors proposed in Japanese Laid-Open Patent Application No. Sho 59-208048 a method which comprises pumping molten low melting point metals into a side wall of a nozzle which pours the molten steel from a ladle into a tundish in a continuous casting process.
Another related art as disclosed in Japanese Laid-Open Patent Application No. Sho 54-94437 teaches to spout solid steel granulars through a hollowed stopper rod positioned just above the tundish nozzle so as to introduce the steel granulars into the molten steel stream by the suction action of the nozzle.
However, these prior arts have the following problems. Thus in the art disclosed in Japanese Laid-Open Patent Application No. Sho 59-208048, as the molten low melting point metal is delivered through a pipe into a side wall of the tapping nozzle, a certain pressure is required and pressure means such as a pump is necessary, which will increase the capital cost and maintenance cost, and further as the molten metal is pumped under pressure, there is a danger that the molten low melting point metal leaks through the joint between the pipe and the side wall of the nozzle.
Meanwhile the art disclosed in Japanese Laid-Open Patent Application No. Sho 54-94437 has the problem that a uniform mixture of molten steel cannot be obtained because the steel granulars are added to the molten steel through the hollowed stopper rod of the tundish to cool and solidify the molten steel.